Flooding in Cities

Cities are complex, populous and interdependent systems which are extremely vulnerable to threats from natural hazards. Recent years have witnessed a sprawling metropolis which has fostered a large build up of highly vulnerable development (Burby, 2006). There is growing concern about placing these compact urban forms in harm's way as new urban developments are aggravating the growing risk to hazards, by adding even higher density developments than in the past. High-density developments place more people, residential and commercial buildings and infrastructure at risk if hazards are not anticipated and hazard mitigation is not prioritised. Does this trend mean that the worlds cities that accommodate a vast proportion of the population are becoming less disaster-resilient? 

Flooding in Northern England following torrential rain over the Christmas period

In terms of flooding, average economic losses alone exceed $6 billion in the US and these losses have been rising relative to increases in population! The flooding caused by Hurricane Katrina which caused over $200 billion in losses was the most costly natural disaster in US history. Coastal cities such as Bay St Louis, Pass Christian and Waveland have increased exposure to catastrophic losses from disasters and thus were nearly obliterated. Should this mass flooding serve as reminder to densely populated coastal populations around the world of their vulnerability to natural hazards?

The UK was subject to flooding across over the Christmas period: claiming lives, causing evacuations, power cuts and chaos, the costs of being under prepared for natural hazards is huge. The Environmental Agency has released another warning stating that Britain faces another month of flood and that there is 'no end in sight' to the bad weather which has disrupted festivities and already brought misery to more than 10,000 people nationwide. The following table shows the flood warnings in place across the UK as of 30/12/2013 at 12:07.

A study by Berke et al. revealed that local governments in the US give more attention to New Urban developments in applying land use regulatory and incentive techniques, public participation initiatives and technical assistances. They have been criticised for being reluctant to anticipate future risks and giving less attention to the use of non-structural mitigation techniques and storm water BMPs. Experts have suggested that more attention needs to be placed on non-structural hazard mitigation in order to counter the building of high-density developments in flood-prone areas in cities across the globe. High-quality planning would mean that attention would be paid to issues such as hazard mitigation thus losses from flooding would be lower in communities that prepare and implement plans for urban development. However this is frequently ignored and continues to occur - a clear example of this are the huge slum developments that are increasingly vulnerable to flooding. This paper looks into the responses of slum dwellers affected by flooding in India.

Looking into the future, hazard mitigation standards such as Smart Code will play a more important role. They prioritise design features such as pedestrian orientated streets, mixed land uses as well as protection of environmentally sensitive areas given their critical mitigation services. However in order to rely on environmentally sensitive natural systems such as wetlands, sand dunes and mangroves to manage and store flood waters, they must be protected. 

New urban developments are laboratories within which we can cautiously experiment with hazard mitigation and environmental protection strategies. They are a living test of how to deal with emerging hazards such as sea level rise and more intense weather events linked to global climate change.

Next time I will look at ways in which cities are acting to mitigate the impact of flooding via levees, storm surge barriers, etc. In the meantime, if you can't get enough of climate change adaptation strategies and flood risk reduction in cities be sure to check out this paper. 

A Lack of Ambition in Disaster Research?

Whilst doing some research for my next post on floods, I came across this extremely engaging post by John Twigg (2013) in his blog on disaster risk reduction:

Words, deeds or institutions?

I thought it was worth highlighting since it provides a refreshingly honest appraisal of some academic work and standards. Twigg suggests that the authors and the journal should seek greater ambition and quality in the work they are producing and publishing respectively. 

How true does this hold for other journals and fields? It would be interesting to hear your thoughts….

Floods: Where Will the Water Lie?

My next few posts are going to be based around flooding. The posts will be interspersed with commentary around the current state of research, evidence of flooding through the Holocene, and what the future holds for human populations facing the flood hazard.

To kick things off here's a video of the destructiveness of floods when they are at their most severe. The Pakistan floods of 2010 were some of the worst in recent years. When you watch the video, start to think about what may cause such a set of events, how can we protect ourselves against the power of water, and what might the implications of anthropogenic climate change be?

Hurricanes and Climate Change

This post is intended to provide an overview of climate change and tropical cyclones. I’m going to start this post with some commentary to set the scene:

The detection and attribution of the possible effects of anthropogenic climate change on tropical cyclones is one of the most controversial topics in present-day science. The increase in tropical cyclone numbers in the Atlantic since the mid 1990s, combined with the devastating impacts of individual hurricanes such as Katrina in 2005, has let to an urgent examination of trends in the available tropical cyclone data to see if these can be explained by man’s effect on the climate.

Walsh et al (2009)

Observations through early October 2006 show that we have so far experienced an average Atlantic basin hurricane season. August had substantially below-average activity (only 45% of average) while September had above-average activity (about 140% of average). US landfall has been well below average. No hurricanes have made landfall along the US coastline this year. This has occurred in only 18 percent of the hurricane seasons since 1945. In an average year about 90 percent of the seasonal average NTC of 100 occurs by 11 October. 

Gray (2006)

El Niño years typically have the following tropical Atlantic conditions:

1.     stronger than normal 200 mb (~12 km) zonal winds (positive U),

2.     dryer middle tropospheric moisture conditions (negative q – specific humidity),

3.     somewhat higher than average sea level pressure anomalies (positive SLPA),

4.     somewhat higher than average sea surface temperature anomalies (positive SSTA).

Gray (2006)

Large amplitude fluctuations in the frequency and intensity of tropical cyclones greatly complicate both the detection of long-term trends and their attribution to rising levels of atmospheric greenhouse gases. Trend detection is further impeded by substantial limitations in the availability and quality of global historical records of tropical cyclones.

Knutson et al (2010)

But what is the situation now?

The IPCC (2013) provide a stark overview:

Warming of the climate system is unequivocal, and since the 1950s, many of the observed changes are unprecedented over decades to millennia. The atmosphere and ocean have warmed, the amounts of snow and ice have diminished, sea level has risen, and the concentrations of greenhouse gases have increased.

Of critical importance for tropical cyclones:

Ocean warming dominates the increase in energy stored in the climate system, accounting for more than 90% of the energy accumulated between 1971 and 2010 (high confidence). It is virtually certain that the upper ocean (0−700 m) warmed from 1971 to 2010 (see Figure SPM.3), and it likely warmed between the 1870s and 1971.

And vitally:

Human influence on the climate system is clear. This is evident from the increasing greenhouse gas concentrations in the atmosphere, positive radiative forcing, observed warming, and understanding of the climate system.

So what does all this mean for the future of tropical cyclones?

The ambiguity of scientific literature investigating linkages between climate change and tropical cyclones is clear. Few papers agree on projected characteristics of storm systems in terms of their frequency and intensity. Much of the research suggests that regional changes will be of chief importance and as a result there is an absence of a generally agreed framework or theory regarding the impact of anthropogenic climate change on hurricanes.

If anthropogenic climate change causes an increase in hurricane activity, shouldn't we see positive trends over, say, the last 100 years? 

When accounting for missing data over that time period, statistical tests reveal that relative to the variability in the dataset, a correlation is not significantly distinguishable form zero. Hurricane numbers were high enough in the 1960s for a couple of decades that there is no significant positive trend from that period in time. Landfalling hurricanes even show a slight negative trend through time. More interesting, perhaps, is the apparent increase in the number of major hurricanes over this period - where major hurricanes are considered Category 4/5. However, the storms that do occur in the warmer climate simulation are more intense on average than those in the control (present day) simulation. 

The 'facts' are:

• Globally there has been no increase in tropical cyclone frequency over the past few decades (e.g. Webster et al, 2005; Elsner and Kocher, 2000).

• Storm frequency has not tracked recent tropical climate trends (Pielke Jr et al, 2005).

• The metrics of tropical cyclone intensity are varied and may be limited by data.

• While observations of tropical and subtropical sea surface temperature have shown an overall increase of about 0.2°C over the past ~50 years, there is only weak evidence of a systematic increase in potential intensity (Bister and Emanuel,2002; Free et al, 2004).

• Other authors suggest the intensity of Atlantic tropical cyclones is rising dramatically.

Elsner (2006) presents an interesting report on two competing hypotheses surrounding causes of increasing Atlantic hurricane activity. One hypothesis, known as the climate change hypothesis, suggests increases in greenhouse gases and their associated changes in eradicative forcing can cause SST in the Atlantic to rise through higher global temperatures. On the other hand, the Atlantic Multidecadal Oscillation (AMO) hypothesis suggests that natural ocean circulation changes in the Atlantic drive hurricane season SST, leading to changes in hurricane frequency and global temperatures. The common agenda in both competing theories is that regional SST is crucial in the genesis of tropical cyclones. The main discussion point is the interconnectivity between global temperatures and SST in the Atlantic. The jury is still out and the challenge remains to find the detailed causal mechanism between climatic change and tropical cyclone activity.

One of the most important pieces of research on the subject has come from Knutson et al (2008) in their letter to Nature. The authors present the results of a study investigating the changes in large-scale climate through the 21^st century using an ensemble of global climate models and describe that Atlantic hurricane frequencies reduce. The results ‘do not support the notion of large increasing trends in either tropical storm or hurricane frequency driven by increases in atmospheric greenhouse-gas concentrations’.

Monster Cyclone Haiyan

Following on from my post about tropical cyclones in the 'Past, Present and Future', I am now going to discuss the recent cyclone Haiyan in the Philippines. One month after Haiyan, the death toll stands at 5,924 and 1,779 people are still missing as per government figures released last Sunday.

'Galvanised iron sheets were flying just like kites' 

(Mai Zamora from World Vision).

Haiyan was one of the strongest typhoons ever to hit land with winds of up to 320 km/h (BBC News). Sustained wind speeds of that extent are what we would expect of a category five hurricane. These winds caused waves as high as 15 metres and up to 400mm of rain in some places. These extreme conditions caused destruction that was extensive and devastating.

Figure 1: Satellite image as Haiyan approached the Philippines

In terms of the damage to infrastructure, the storm caused widespread damage. Reports of building being ripped apart, flash floods and landslides were rampant. The country was in a state of chaos whereby schools and offices were closed, flights were suspended and soldiers were entrusted with rescue and relief operations. Furthermore, the storm meant that power and communication lines were cut to some areas.

Haiyan affected millions of people, 12 million according to officials. Even the capital felt the force of the storm despite being far away from the eye of the storm, however other cities weren't as fortunate. Raging across Leyte and Samar, it turned roads into rivers and left Cebu city devastated, home to over 2.5 million people. The black arrow in figure 2 indicates the path of the storm. It suggests that the storm passed through some of the most densely populated parts of the Philippines therefore affecting an extensive proportion of the population. 

Figure 2: population density of the Philippines

The Philippines has experienced numerous super typhoons over the past decades. Haiyan was the 25th to enter the territory this year. Scattered along the worlds most active typhoon belt, there are plentiful supplies of warm water and moist air to provide the energy to kick start super storms. Despite these factors concerning the formation of a cyclone, Haiyan has shown a number of unusual features. 

As explored in my previous post, the walls of the storm that normally rotate around the eyes are replaced as it moves, often weakening the wind speed. In the case of Haiyan, this didn't happen. Further to this, the upwelling of cold water which would usually serve to reduce the energy of a storm did not take place as Haiyan was travelling so quickly. These two factors meant that Haiyan was unique in nature. 

Figure 3: typhoon Haiyan relief effort

In a blog post by Jeff Masters, the damage is described as 'perhaps the greatest wind damage any city on Earth has endured from a tropical cyclone in the past century'. Figure 3 gives just a snap shot of the damage. 

The question is: will this trend of monster cyclones gain momentum in the future? Does anthropogenic climate change have a role in the increasing frequency and severity of so called 'natural' disasters? Find out more on hurricanes through the Anthropocene next time. 

World’s Worst Earthquakes Caused By Man

This short article in the Sunday times caught my attention whilst I was reading about current trending stories about Nelson Mandela and Nigella Lawson. 'Scientists have suggested some of the worlds largest and most deadly recent earthquakes were not natural disasters at all - but were instead caused by human activities' (Jonathan Leake, Science Editor). To read the whole article click here.

Figure 1: The Sichuan earthquake in China

The article has emerged following findings from a global study of hundreds of earthquakes by Klose whose research in the Journal of Seismology identified 92 large earthquakes which could be linked to human impact. He published a paper in 2010 titled 'Human-Triggered Earthquakes and Their Impacts on Human Security' which is also worth looking at. 

This is extremely interesting and relevant. So far in my blog, I have looked at the science of different individual natural hazards and the characteristics of our world population in terms of size and location. In the next few weeks I will begin to draw links between hazards and population. This article is perfect to get the ball rolling. 

Tropical Cyclones: Past, Present and Future

After looking into extra tropical cyclones in some detail, I would like to move on to another important climate hazard, tropical cyclones, in terms of the science, response to a changing climate and their impact on people. The Met Office describes Tropical cyclones as as amongst the most powerful and destructive meteorological systems on earth. I have produced two simple graphics using data from Munich Re (2013). The first shows the costliest natural disasters in term of monetary loss whilst the second depicts the same story for human fatalities. Costs consequent of tropical cyclone appear high up on these lists.  

The awesome might of these tropical systems can be seen in the video below which shows scenes from the aftermath of hurricane sandy, the second costliest natural disaster of all time behind the recent Japanese Tsunami. Tropical storms have the potential to wreck cities, livelihoods and cause long-term demographic, social and political change. 

The mechanics of tropical cyclones are relatively well understood. It is generally accepted that there are 6 critical antecedent conditions which must prevail in order for such systems to develop, intensify and in turn propagate. Of critical importance is the presence of warm ocean waters, at least 27 degrees centigrade. Related to warm waters is the depth of the thermocline, that is, the depth at which there is a sharp transition from warmer waters to deeper cold water. A deep thermocline means that if a tropical cyclone were to develop the associated turbulence only returns more warm water to the surface of the ocean and hence continues to fuel the genesis process In contrast, a shallow thermocline might mean that the strong winds of a hurricane bring cold waters up from below and self prohibit serious intensification. In terms of Atlantic systems, the position of the loop current may play an important role in allowing cyclones to intensify en route to Louisiana or Florida. (Will be considered in more detail in a later post.) The second key precursory condition is an atmosphere which is unstable. That is, an atmosphere which cools significantly wit height. Other requirements are moist layers in the mid-troposphere, a minimum distance of 500km from the equator (so that there is sufficient Coriolis Force to spin the cyclone), pre-existing near-surface disturbances, and low levels of vertical wind shear. The figure below illustrates the tracks of all historical Atlantic tropical storms. The tropical Atlantic source is evident as a favourable location for genesis, as is the northwards trajectory where storms pick up the Gulf Stream's warm ocean waters. Furthermore, It is clear that coastal regions bare the brunt of the tropical storm's force and that inland regions are relatively secure. 

The next image documents the anatomy of a hurricane. The eye which is roughly 20-50 km in diameter is found at the centre of the system. The eye wall immediately adjacent presents very strong winds, intense rain and thunderstorms. It is important to note that the direction of winds at the bottom of the system are in the opposite direction of rotation than at the top. Rising air at the centre of the storm is opposed by subsidence on the outer edges. Energy for intensification of the hurricane comes from sensible heat and latent heat as vapour changes state which creates high pressure, divergence aloft and lower pressure at the surface. The increased pressure gradient generates strong surface level winds. Just as there are conditions that favour the development of tropical storms, there are conditions that inhibit their formation too. Strong trade wind inversion, cooler sea surface temperatures and strong upper level winds play a role. 

Storms can leave distinctive sedimentary deposits in shallow marine deposits and coastal lagoons. Davis et al (1989) inferred that 'hurricanes produced graded or homogeneous facies of sand, shell gravel, and mud found in predominantly clastic sediments of late Holocene age in coastal lagoonal bays of Florida'. The hurricane signature is largely a consequence of their immense energy. As such, they can act as geomorphic agents wit the potential to cause coastal landform changes, particularly when hurricanes reach categories 4 or 5 when the winds get particularly strong. Liu and Fearn (1993) suggest that sediment cores from Lake Shelby on the Alabama coast show more tranquil climatic regimes prior to 3.2 ka BP as a result of a complete absence of sand layers in the cores. Furthermore, the authors propose that a more mesic climate post 3.2 ka BP was responsible for an unusual incursion of hurricanes into southwest Texas. Donnelly and Woodruff (2007) examined the millenial scale variability of Caribbean hurricane activity by reconstructing hurricane-induced overwash events from Laguna Playa Grande, Puerto Rico. The data reveals large scale fluctuation in the frequency of intense hurricanes. Relatively frequent occurrences of intense hurricanes are shown to have occured between 5.4 ka BP and 3.6 ka BP with the exception of a short quiet period from 4.9 ka BP to 5.1 ka BP. Following this period is a spell of quiet activity from 3.6 ka BP to 2.5 ka BP. Another active spell occurs between 2.5 ka BP and 1 ka BP. This is depicted in the figure below. A key future research goal is to relate these activities more reliably to past SSTs based on coral or sediment records. Such issues are at the centre of current debate on the impact of changing SSTs in the face of anthropogenic climate change will have on future hurricane activity. One this that is more certain, however, is that there is a link between La Nina years and intensified NAHU activity and supression during El Nino. This is chiefly due to increased vertical wind shear (noted above as a controlling factor) in strong El Nino seasons. 

That's it for now, later this week I hope to discuss a case study of the recent cyclone to have devastated the Philippines as well as prompting a discussion on the future for hurricanes in the face of anthropogenic climate change. The latest scientific hurricane research will be debated and participation is encouraged! 

YOLO

YOLO. True. But as we move further into the Anthropocene, we are living much longer than our ancestors did a few thousand years ago. This definitely warrants a few minor diversions:

- a music video 

- 'How Long Will I Live?' a website where you can calculate how many years you have got left

- an insight into the use of the acronym on twitter

Let's get back to life expectancy. My post will talk about the general upward trend of life expectancy, what that means for population and explore reasons why we are living longer.

What's trending?

A trend in the life expectancy of humans during the past thousand years has been characterised by a slow and steady increase. Epidemics, famines and warfare were to blame for frequently upsetting this upward trend with volatile death rates however this curtailed in the mid-19th century. Why? Due to improved living conditions, advances in public health and medical interventions. 

The figures below suggest the changing picture of mortality. It is crystal clear that life expectancy is climbing and projections suggest that this trend will continue to increase. Only 50 years ago, life expectancy was just 68 years of age, now we can expect to live past 79. That's an increase of about 16%. A key indicator of the dramatic change in life expectancy is the growth of people aged over 100 in our society. In terms of centenarians per million, by 2030 estimates predict anything between 515 and 3,500. Now that's a LOT of letters from the queen. 

Figures 1 and 2 use data taken from RMS

We must be careful however when generalising life expectancy across the globe, as it can be different within the same city. Places just a few miles apart geographically have life expectancy spans varying by years. The diagram gives an example of a small area in London along the Jubilee Line. It suggests that men living in Westminster can expect to live at least 4 years longer than those in Canning Town!

Why the upward trend?

The greatest advance of medical science in history has helped to push life expectancy through the roof. Let's break this down and look at the major social changes year by year: 

	Lifestyle	Medical	Health Economics
1950	Two thirds of adults smoke; smoking linked to cancer proved in 1954	Discovery of structure of DNA. Penicillin in mass production	Healthcare insurance offered by US employers. Universal health services set up in Europe
1990	Tobacco companies settle law suit for $206 bn; tobacco advertising banned. Food calorie labelling	Clinical trials prove statins cut heart deaths, 5% of adults take statins by 1998; heart attack mortality rate 80%	Healthcare expenditure averages $2000 per person. Consolidation of drug industry into giant pharmaceutical companies
2010	A quarter of adult population smokes; smoking bans in many countries	Lipitor becomes world’s best-selling drug; stem cell transplants; heart attack mortality rate 40%	Health expenditure 9% of GDP (average $3000 per person)
2030 – What If?	Nobody smokes; obesity wave halted; fitness levels higher than today	Cancer becomes managed disease	20% of GDP spent on medicine

This obsession with longevity feeds into our every day lives. Headlines such as 'sex adds years to your life' this century, have triggered a mad outburst in people searching for activities, foods, diets and wild and wondrous ways to live longer. wikiHow gives you an opportunity to live a very long life just by having a read of this article.  

Shocks to the system?

The last 20 years have seen unprecedented mortality improvement. Medical advances have been fundamental in this undeniable upward trend in life expectancy, however there have been numerous 'mortality shocks' which work against this trend. 

• Asian Flu pandemic 1957/8 kills up to 4 million people worldwide 
• HIV/AIDs emerges in 1980s, reaching 8 million cases by 1990
• SARS emerges as a new infectious disease in 2002
• Terrorist attack 2001 kills 3000
• Avian flu outbreaks from 2006 puts world on pandemic alert

Pandemics are a threat to life expectancy now, but will they always be? Will technological fixes make us immune to pandemics in the future? What type of mortality shocks will gain momentum in years to come? What impact will climate hazards have on life expectancy? The recent Philippines typhoon alone is expected to have claimed approximately 2000 lives. 

The 5 'catastrophe perils' in the diagram below are shocks which have the potential to cause excess mortality in the future. Infectious diseases have featured throughout history, however natural hazards and mass-casualty terrorism are factors which have merely been considered as a threat to life expectancy in the past.

RMS include 'natural hazards' in their life-risk models, a factor which could start to gain momentum. Unprecedented urbanisation to coastal areas (explored in a previous post) means thousands of people are vulnerable to the increasing frequency of climate hazards. Do climate hazards have the capacity to reverse the upward trend of life expectancy? That is the question.

NAO in the News

Related to much of my previous discussion, I have found this great article in the Daily Mail.

'Why is it so cold? Simple... it's the North Atlantic Oscillation - and it's got a bit stuck'

Well worth a read! 

Question Time

My recent posts have raised some interesting questions. I thought I'd tackle them as a post in itself such was their relevance.

Ok here goes, question number 1:

"Cool post Isabela. It seems that the NAO is really important in determining the formation of extra-tropical cyclones. You mentioned that anthropogenic climate change might be influencing the NAO. Despite the lack of unequivocal evidence, has a causal mechanism by which climate change influences the NAO been identified?"

Thanks for your question. A causal mechanism for NAO variability is a complex and controversial topic. These have been a number of interpretations and many have used GCM simulations under different parameterisation. Robertson et al (2000) suggest SST distribution plays a role in modulating both interannual variability and regional modes. It is certainly clear that SSTs are changing in the midst of a changing climate, too.

Others have suggested anthropogenic climate change may influence the strength of the stratospheric vortex and in turn the NAO. In both cases, though, predictive models lack skill and robustness. Modelling improvements will be key in attributing a causal mechanism.

Question 2:

"Looking at the NAO index diagram you have there also appears to be a prolonged positive phase between 1900 and 1920, were there an increased storm frequency then as well?"

Here's a reminder of exactly what we mean by a positive phase...

Firstly, an important point to note is that the term North Atlantic Oscillation was only coined in the 1920s by Sir Gilbert Walker. Walker's 1924 writings eluded to the traditional description of the NAO through correlation between pressure anomalies between the sub tropics and Iceland. Consequently uncertainty is induced through an incomplete knowledge of the atmospheric system itself, and, secondly, through limited recording of weather events. Nevertheless the period from ~1900 to 1930 did represent a period where a positive phase persisted over multiple winters. I recommend this paper by Andrade et al (2008). The authors look at historic records of storm frequency from the Azores region. This extract summarises some important conclusions from the paper:

"A number of major periods of contrasting storminess were identified. The first period (AD 1836–1870) is characterized by a distribution with two maxima and low yearly storm frequency (2–3 storms/yr). A second period extends between AD 1870 and 1920 and corresponds to an irregular distribution of storm frequency that decreases in time, maximum values reported in 1879 and 1886; the first years of this period record an abrupt peak of storminess (> 8 storms/yr) that drops to 1 storm/yr by the end of the 1911–1920 decade. A third period corresponds to the decades of AD 1920–1940 with uniform distribution and low storm frequency. After 1940 (period 4) a general trend of increasing storminess emerges (2–3 to 4–5 storms/yr on average) with maxima occurring in the middle of the 1980s."

A Mini Case-Study of Cyclone Lothar

Last post I looked at the science of extra-tropical cyclones, today I will explore the damages that these cyclones can cause through a mini case-study of Cyclone Lothar. 

Cyclone Lothar was one of three extreme storms over Europe in December 1999. In total they claimed more than 130 lives and caused about 13 billion euros worth of total economic losses. Lothar was the second storm event which caused a trail of destruction from north-western France to Southern Germany and Switzerland. The largest damages occurred around Paris' well-known tourist sites with gusts reaching wind speeds of almost 50 ms. Damages to forests, buildings and infrastructure was widespread. In France public life was severely disrupted due to power outages, infrastructural damage caused substantial economic losses and over 10,000 trees were uprooted within hours. The photo below was taken in the area following the storm and hints at the extent of damage caused by countless uprooted trees:

In Switzerland, more than 12 million cubic metres of timber were damaged, approximately 3% of the growing stock of the whole country: that's the equivalent of an entire years supply of wood at Ikea's current usage (they account for 1% of all wood used commercially around the world). The economic costs were sky high, amounting to 1.4 billion euros. Transport facilities, cable cars, telecommunication services and the Swiss electricity network were also heavily affected. In terms of human costs, 14 people were killed during the event, mostly from falling trees or bricks and a further 15 people died during clearance work in the forests. 

Here are a few bullet points to help summarise the key points mentioned above:

• formed on Christmas Day of 1999 
• affected Western Europe 
• highest gust recorded 161 mph 
• 110 fatalities

Make sure you check out this video taken from an apartment block on the coast of Vevey (Switzerland) for footage of the storm in action! For those kill joys who didn't click the link, I'm giving you a second chance. Ready, steady - GO!

Can the occurrence of these storms be taken as fuel for discussions on anthropogenic climate change? How does this compare to St Jude only a few days ago? As the worlds population moves into cities along its coasts are we making ourselves more vulnerable? 'Self-regulating machine'? Is it starting to make sense yet? No? Stay with me. Hold that thought, I will come back to it. Later.

Extra-tropical cyclones: The science

Severe extra-tropical cyclones usually form in the North Atlantic basin and represent the dominant feature of the mid-latitudes (Ulbrich et al, 2009). The graphic below from Risk Management Solutions (RMS, 2006)  documents key historical European windstorm events with their associated economic losses. It is striking that developed, heavily populated countries appear so exposed.  The UK, France, Germany, Poland and other significant European countries have suffered from significant losses in the past, and seem sure to be struck again. Cluster events can be particularly damaging, and the events of 1990 and 1999 provide a reminder of these dangers. But what exactly are extra-tropical cyclones? 

The mechanics of extra-tropical cyclones take a very different form to, for example, tropical cyclones, during the processes of genesis, intensification and propagation. Extratropical windstorms tend to be fast moving and broad reaching storms which result in low level of damage at individual sites. The breath of the storms, which can reach 2,000km in diameter, mean losses have the potential to accumulate to significant sums.  In addition to strong winds, heavy precipitation and sharp temperature changes are associated with these large frontal systems. The extra-tropical transition at the midlatitudes is usually a product of low pressure systems where energy is gained from meridional temperature gradients created by polar air masses and subtropical air masses (Harr et al, 1999; Jones et al, 2003). The gradient is greatest during winter and most sever storms tend to develop over this period. The track of the storms is controlled by the position of the Polar Jet, which has the capacity to shift the track north or south.  Ulbrich et al  (2009) suggest that many studies show overall cyclone frequencies to have decreased over the period ~1980-2000. In contrast, however, the average intensity of such storms is shown to have increased with some illustrating an eastwards extension of deep lows in wintertime over the last 50 years. 

Of chief importance in producing conditions favourable to the development of extra-tropical cyclones is the North Atlantic Oscillation (NAO). The NAO is a 'hemispheric meridional oscillation in atmospheric mass with centers of action near Iceland and over the subtropical Atlantic' (Visbeck et al, 2001). The figure below shows its changes in index between 1860 and 2000. The index is defined as the difference between the polar low and subtropical high from December to March. A positive NAO index reflects a stronger than normal pressure gradient and hence more and stronger winds over the North Atlantic ocean basin. Over the last 30 years the NAO has seen an unprecedented trend towards a more positive phase. Incidentally the big cluster storms of 1990 and 1999 occurred over this period. There is an increasing argument that anthropogenic climate change might be influencing this natural mode of atmospheric variability. GCMs do not yet give an unequivocal answer on this viewpoint however. 

Ulbrich et al (2009) support the view that anthropogenic climate change is taking hold, stating that a major result from an ensemble of different models forced with different greenhouse gas concentrations is that extreme extratropical cyclones increase in number in the winter months (when NAO has been at its peak), whilst the total number is slightly reduced in both the norther and southern Hemispheres. 

Bonazzi et al (2012) comment that the damages associated with the European Windstorm peril are amongst the costliest natural perils in Europe. Knowledge on storm development, spatial structure and future frequency and intensity will be crucial in minimising the risk. 

Now you know a little more about the Windstorm peril, the next post will take a look at the impact they can have on humans and our preparedness in a little more detail.